U.S. patent number 5,945,034 [Application Number 09/204,241] was granted by the patent office on 1999-08-31 for organic positive temperature coefficient thermistor.
This patent grant is currently assigned to TDK Corporation. Invention is credited to Tokuhiko Handa, Yukie Yoshinari.
United States Patent |
5,945,034 |
Handa , et al. |
August 31, 1999 |
Organic positive temperature coefficient thermistor
Abstract
An organic positive temperature coefficient thermistor comprises
a thermoplastic polymer matrix, a low-molecular organic compound,
and conductive particles, each having spiky protuberances thereon.
The low-molecular organic compound has a melting point of
40.degree. C. to 100.degree. C. The conductive particles are
interconnected in a chain form.
Inventors: |
Handa; Tokuhiko (Chiba,
JP), Yoshinari; Yukie (Chiba, JP) |
Assignee: |
TDK Corporation (Tokyo,
JP)
|
Family
ID: |
18408296 |
Appl.
No.: |
09/204,241 |
Filed: |
December 3, 1998 |
Foreign Application Priority Data
|
|
|
|
|
Dec 4, 1997 [JP] |
|
|
9-350108 |
|
Current U.S.
Class: |
252/511; 219/541;
252/518.1; 264/104; 252/510; 252/513; 264/234; 264/347; 264/105;
252/512; 219/547; 219/546; 219/553; 338/22R |
Current CPC
Class: |
H01C
7/027 (20130101); G01K 7/223 (20130101) |
Current International
Class: |
H01B
1/24 (20060101); H01B 1/20 (20060101); H01B
1/22 (20060101); H01C 7/02 (20060101); H01B
1/00 (20060101); G01K 7/16 (20060101); G01K
7/22 (20060101); H01B 001/00 (); H01B 001/20 ();
H01B 001/22 (); H01B 001/24 () |
Field of
Search: |
;252/500,510-513,518,519 |
References Cited
[Referenced By]
U.S. Patent Documents
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|
|
5378407 |
January 1995 |
Chandler et al. |
|
Foreign Patent Documents
Primary Examiner: Gupta; Yogendra
Assistant Examiner: Hamlin; Derrick G
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Claims
What we claims is:
1. An organic positive temperature coefficient thermistor
comprising a thermoplastic polymer matrix, a low-molecular organic
compound, and conductive particles having spiky protuberances
thereon.
2. The organic positive temperature coefficient thermistor
according to claim 1, wherein said low-molecular organic compound
has a melting point of 40.degree. C. to 100.degree. C.
3. The organic positive temperature coefficient thermistor
according to claim 1, wherein said conductive particles, each
having spiky protuberances, are interconnected in a chain form.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an organic positive temperature
coefficient thermistor that is used as a temperature sensor or
overcurrent-protecting element, and has PTC (positive temperature
coefficient of resistivity) characteristics that its resistance
value increases with increasing temperature.
An organic positive temperature coefficient thermistor having
conductive particles dispersed in a crystalline polymer has been
well known in the art, as typically disclosed in U.S. Pat. Nos.
3,243,753 and 3,351,882. The increase in the resistance value is
believed to be due to the expansion of the crystalline polymer upon
melting, which in turn cleaves a current-carrying path defined by
the conductive fine particles.
An organic positive temperature coefficient thermistor can be used
as a self regulating heater, an overcurrent-protecting element, and
a temperature sensor. Requirements for these are that the
resistance value is low at room temperature in a non-operating
state, the rate of change between the room-temperature resistance
value and the resistance value in operation is sufficiently large,
and the resistance value change upon repetitive operations is
reduced. In applications such as temperature sensors, the
temperature vs. resistance curve hysteresis should be reduced.
To meet such requirements, it has been proposed to incorporate a
low-molecular organic compound such as wax in a polymer matrix.
Such an organic positive temperature coefficient thermistor, for
instance, includes a polyisobutylene/paraffin wax/carbon black
system (F. Bueche, J. Appl. Phys., 44, 532, 1973), a
styrene-butadiene rubber/paraffin wax/carbon black system (F.
Bueche, J. Polymer Sci., 11, 1319, 1973), and a low-density
polyethylene/paraffin wax/carbon black system (K. Ohe et al., Jpn.
J. Appl. Phys., 10, 99, 1971). Self regulating heaters,
current-limiting elements, etc. comprising an organic positive
temperature coefficient thermistor using a low-molecular organic
compound are also disclosed in JP-B's 62-16523, 7-109786 and
7-48396, and JP-A's 62-51184, 62-51185, 62-51186, 62-51187,
1-231284, 3-132001, 9-27383 and 9-69410. In these cases, the
increase in the resistance is believed to be due to the melting of
the low-molecular organic compound.
One of advantages to the use of the low-molecular organic compound
is that there is a sharp rise in the resistance increase with
increasing temperature because the low-molecular organic compound
is generally higher in crystallinity than a polymer. A polymer,
because of being easily put into an over-cooled state, shows a
hysteresis where the temperature at which there is a resistance
decrease with decreasing temperature is usually lower than the
temperature at which there is a resistance increase with increasing
temperature. With the low-molecular organic compound it is then
possible to keep this hysteresis small. By use of low-molecular
organic compounds having different melting points, it is possible
to easily control the temperature (operating temperature) at which
there is a resistance increase. A polymer is susceptible to a
melting point change depending on a difference in molecular weight
and crystallinity, and its copolymerization with a comonomer,
resulting in a variation in the crystal state. In this case, no
sufficient PTC characteristics are often obtained. This is
particularly true of the case where the operating temperature is
set at 100.degree. C. or lower.
One of the above publications, Jpn. J. Appl. Phys., 10, 99, 1971
shows an example wherein the specific resistance value (.OMEGA.-cm)
increases by a factor of 10.sup.8. However, the specific resistance
value at room temperature is as high as 10.sup.4 .OMEGA.-cm, and so
is impractical for an overcurrent-protecting element or temperature
sensor in particular. Other publications show resistance value
(.OMEGA.) or specific resistance (.OMEGA.-cm) increases in the
range between 10 times or lower and 10.sup.4 times, with the
room-temperature resistance being not sufficiently low.
In many cases, carbon black has been used as conductive particles
in prior art organic positive temperature coefficient themistors
including the above-mentioned ones. A problem with carbon black is,
however, that when an increased amount of carbon black is used to
lower the initial resistance value, no sufficient rate of
resistance change is obtainable. Sometimes, particles of generally
available metals are used as conductive particles. In this case,
too, it is difficult to arrive at a sensible tradeoff between low
initial resistance and a large rate of resistance change.
One approach to solving this problem is disclosed in JP-A 5-47503
that teaches the use of conductive particles having spiky
protuberances. More specifically, it is disclosed that
polyvinylidene fluoride is used as a crystalline polymer and spiky
nickel powders are used as conductive particles having spiky
protuberances. U.S. Pat. No. 5,378,407, too, discloses a thermistor
comprising filamentary nickel having spiky protuberances, and a
polyolefin, olefinic copolymer or fluoropolymer.
However, these thermistors are still insufficient in terms of
hysteresis and so are unsuitable for applications such as
temperature sensors, although the effect on the tradeoff between
low initial resistance and a large resistance change is
improved.
One object of the present invention is to provide an organic
positive temperature coefficient thermistor that shows a reduced
temperature vs. resistance curve hysteresis, makes control of
operating temperature easy, and has both sufficiently low
room-temperature resistance and a large rate of resistance change
between an operating state and a non-operating state. Another
object of the invention is to provide an organic positive
temperature coefficient thermistor that does not only meet such
requirements but can also be operated at 100.degree. C. or
lower.
SUMMARY OF THE INVENTION
Such objects are achieved by the inventions defined below as (1) to
(3).
(1) An organic positive temperature coefficient thermistor
comprising a thermoplastic polymer matrix, a low-molecular organic
compound, and conductive particles, each having spiky protuberances
thereon.
(2) The organic positive temperature coefficient thermistor
according to (1), wherein said low-molecular organic compound has a
melting point of 40.degree. C. to 100.degree. C.
(3) The organic positive temperature coefficient thermistor
according to (1) or (2), wherein said conductive particles, each
having spiky protuberances, are interconnected in a chain form.
In the present invention, the spiky shape of protuberances on the
conductive particles enables a tunnel current to pass readily
through the thermistor, and makes it possible to obtain initial
resistance lower than would be possible with spherical conductive
particles. When the thermistor is in operation, a large resistance
value is obtainable because spaces between the spiky conductive
particles are larger than those between spherical conductive
particles.
In the present invention, the low-molecular organic compound is
molten to achieve the PTC (positive temperature coefficient of
resistivity) characteristics that the resistance value increases
with increasing temperature, so that the temperature vs. resistance
curve hysteresis can be more reduced than that obtained by use of
the polymer matrix alone. Control of operating temperature by use
of low-molecular organic compounds having varying melting points,
etc. is easier than control of operating temperature making use of
a change in the melting point of a polymer.
In this regard, it is noted that JP-A 5-47503 discloses an organic
positive temperature coefficient thermistor characterized by
comprising a crystalline polymer, and conductive particles milled
with said crystalline polymer, each of said conductive particles
having spiky protuberances. U.S. Pat. No. 5,378,407 discloses a
conductive polymer composition comprising filamentary nickel having
spiky protuberances, and a polyolefin, olefinic copolymer or
fluoropolymer. However, these publications are silent about the use
of the low-molecular organic compound, unlike the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features, and advantages of the
present invention will be better understood from the following
description taken in conjunction with the accompanying
drawings.
FIG. 1 is a sectional schematic of one embodiment of the organic
positive coefficient thermistor according to the invention.
FIG. 2 is a temperature vs. resistance curve for the thermistor
element according to Example 1.
FIG. 3 is a graphical view for illustrating how to find the
operating temperature, thereby determining the degree of hysteresis
from a temperature vs. resistance curve.
FIG. 4 is a temperature vs. resistance curve for the thermistor
element according to Example 2.
FIG. 5 is a temperature vs. resistance curve for the thermistor
element according to Example 3.
FIG. 6 is a temperature vs. resistance curve for the thermistor
element according to Example 4.
FIG. 7 is a temperature vs. resistance curve for the thermistor
element according to Comparative Example 1.
FIG. 8 is a temperature vs. resistance curve for the thermistor
element according to Comparative Example 2.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will now be explained in more detail.
The organic positive temperature coefficient thermistor of the
invention comprises a thermoplastic polymer matrix, a low-molecular
organic compound, and conductive particles, each having spiky
protuberances. Preferably, the conductive particles having spiky
protuberances are milled with the thermoplastic polymer matrix with
which the low-molecular organic compound is mixed.
The polymer matrix used may be either crystalline or amorphous with
the proviso that it is of thermoplasticity. To prevent fluidization
and deformation of the polymer matrix due to the melting of the
low-molecular organic compound during operation, however, it is
desired that the melting or softening point of the polymer matrix
be higher than the melting point of the low-molecular organic
compound, preferably by at least 30.degree. C., and more preferably
by 30.degree. C. to 110.degree. C. inclusive. It is then desired
that the melting or softening point of the thermoplastic polymer
matrix be usually 70 to 200.degree. C.
Preferably but not exclusively, the low-molecular organic compound
used is a crystalline yet solid (at normal temperature or about
25.degree. C.) substance having a molecular weight of up to about
1,000.
Such a low-molecular organic compound, for instance, includes waxes
(e.g., petroleum waxes such as paraffin wax and microcrystalline
wax as well as natural waxes such as vegetable waxes, animal waxes
and mineral waxes), and fats and oils (e.g., fats, and those called
solid fats). Components of the waxes, and fats and oils may be
selected from hydrocarbons (e.g., an alkane type straight-chain
hydrocarbon having 22 or more carbon atoms), fatty acids (e.g., a
fatty acid of an alkane type straight-chain hydrocarbon having 22
or more carbon atoms), fatty esters (e.g., a methyl ester of a
saturated fatty acid obtained from a saturated fatty acid having 20
or more carbon atoms and a lower alcohol such as methyl alcohol),
fatty amides (e.g., a primary amide of a saturated fatty acid
having 10 or less carbon atoms, and an unsaturated fatty amide such
as oleic amide, and erucic amide), aliphatic amines (e.g., an
aliphatic primary amine having 16 or more carbon atoms), and higher
alcohols (e.g., an n-alkyl alcohol having 16 or more carbon atoms).
However, these components may be used by themselves as the
low-molecular organic compound.
These low-molecular organic compounds are commercially available,
and commercial products may be immediately used alone or in
combination of two or more.
In the present invention, one object is to provide a thermistor
that can be operated preferably at 100.degree. C. or lower, using
the low-molecular organic compound having preferably a melting
point, mp, of 40 to 100.degree. C. Such a low-molecular organic
compound, for instance, includes paraffin waxes (e.g., tetracosane
C.sub.24 H.sub.50 mp 49-52.degree. C.; hexatriacontane C.sub.36
H.sub.74 mp 73.degree. C.; HNP-10 (trade name) mp 75.degree. C.,
Nippon Seiro Co., Ltd.; and HNP-3 mp 66.degree. C., Nippon Seiro
Co., Ltd.), microcrystalline waxes (e.g., Hi-Mic-1080 (trade name)
mp 83.degree. C., Nippon Seiro Co., Ltd.; Hi-Mic-1045 mp 70.degree.
C., Nippon Seiro Co., Ltd.; Hi-Mic-2045 mp 64.degree. C., Nippon
Seiro Co., Ltd.; Hi-Mic-3090 mp 89.degree. C., Nippon Seiro Co.,
Ltd.; Seratta 104 mp 96.degree. C., Nippon Sekiyu Seisei Co., Ltd.;
and 155 Microwax mp 70.degree. C., Nippon Sekiyu Seisei Co., Ltd.),
fatty acids (e.g., behenic acid mp 81.degree. C., Nippon Seika Co.,
Ltd.; stearic acid mp 72.degree. C., Nippon Seika Co., Ltd.; and
palmitic acid mp 64.degree. C., Nippon Seika Co., Ltd.), fatty
esters (arachic methyl ester mp 48.degree. C., Tokyo Kasei Co.,
Ltd.), and fatty amides (e.g., oleic amide mp 76.degree. C., Nippon
Seika Co., Ltd.). Use may also be made of wax blends which comprise
paraffin waxes and resins and may further contain microcrystalline
waxes, and which have a melting point of 40 to 100.degree. C.
The low-molecular organic compounds may be used alone or in
combination of two or more although depending on operating
temperature and so on.
The thermoplastic polymer matrix used herein, for instance,
includes:
i) polyolefin (e.g., polyethylene);
ii) copolymer composed of monomer units derived from one or two or
more olefins (e.g., ethylene, and propylene) and an olefinic
unsaturated monomer having one or two or more polar groups (e.g.,
an ethylene-vinyl acetate copolymer), polymethyl (meth)acrylates,
and EVA;
iii) halogenated vinyl and vinylidene polymers (e.g., polyvinyl
chloride, polyvinylidene chloride, polyvinyl fluoride, and
polyvinylidene fluoride);
iv) polyamide (e.g., 12-nylon);
v) polystyrene;
vi) polyacrylonitrile;
vii) thermoplastic elastomer;
viii) polyethylene oxide, and polyacetal;
ix) thermoplastic modified cellulose;
x) polysulfones; and
More specific reference is made to high-density polyethylene (e.g.,
Hizex 2100JP, Mitsui Petrochemical Industries, Ltd., and Marlex
6003, Phillips Petroleum Co.), low-density polyethylene (e.g.,
LC500, Nippon Polychem Co., Ltd., and DYNH-1, Union Carbide Corp.),
medium-density polyethylene (2604M, Gulf Oil Corp. ),
ethylene-ethyl acrylate copolymer (e.g., DPD 6169, Union Carbide
Corp.), ethylene-acrylic acid copolymer (EAA 455, Dow Chemical
Co.), hexafluoroethylene-tetrafluoroethylene copolymer (e.g., FEP
100, Du Pont), and polyvinylidene fluoride (e.g., Kynar 461,
Penvalt). It is preferable that such thermoplastic polymers have
preferably a molecular weight of about 10,000 to 5,000,000 as
expressed by weight-average molecular weight, a melting or
softening point of 70 to 200.degree. C. as already mentioned, and a
melt flow rate of 0.1 to 30 g/10 minutes as defined by ASTM
D1238.
These thermoplastic polymers may be used alone or in combination of
two or more. Although it is preferable that the polymer matrix is
composed only of such a thermoplastic resin as mentioned above
(which resin may be crosslinked), it is understood that the polymer
matrix may optionally contain elastomers or thermosetting resins or
their mixture.
The conductive particles used herein, each having spiky
protuberances, are each made up of a primary particle having
pointed protuberances. More specifically, a number of (usually 10
to 500) conical and spiky protuberances, each having a height of
1/3 to 1/50 of particle diameter, are present on one single
particle. The conductive particles are preferably made up of nickel
or the like.
Although such conductive particles may be used in a discrete powder
form, it is preferable that they are used in a chain form of about
10 to 1,000 interconnected primary particles. The chain form of
interconnected primary particles may partially include primary
particles. Examples of the former include a spherical form of
nickel powders having spiky protuberances, one of which is
commercially available under the trade name of INCO Type 123 Nickel
Powder (INCO Co., Ltd.). These powders have an average particle
diameter of about 3 to 7 .mu.m, an apparent density of about 1.8 to
2.7 g/cm.sup.3, and a specific surface area of about 0.34 to 0.44
m.sup.2 /g.
Preferred examples of the latter are filamentary nickel powders,
some of which are commercially available under the trade names of
INCO Type 255 Nickel Powder, INCO Type 270 Nickel Powder, INCO Type
287 Nickel Powder, and INCO Type 210 Nickel Powder, all made by
INCO Co., Ltd., with the former three being preferred. The primary
particles have an average particle diameter of preferably at least
0.1 .mu.m, and more preferably from about 0.5 to about 4.0 .mu.m
inclusive. Primary particles having an average particle diameter of
1.0 to 4.0 .mu.m inclusive are most preferred, and may be mixed
with 50% by weight or less of primary particles having an average
particle diameter of 0.1 .mu.m to less than 1.0 .mu.m. The apparent
density is about 0.3 to 1.0 g/cm.sup.3 and the specific surface
area is about 0.4 to 2.5 m.sup.2 /g.
In this regard, it is noted that the average particle diameter is
measured by the Fischer subsieve method.
Such conductive particles are set forth in JP-A 5-47503 and U.S.
Pat. No. 5,378,407.
Referring to the mixing ratio between the thermoplastic polymer
matrix and the low-molecular organic compound, it is preferable
that the low-molecular organic compound is used at a ratio of 0.2
to 4 (by weight) per thermoplastic polymer. At such a weight ratio
it is possible to take full advantage of the invention. When this
ratio becomes low or the amount of the low-molecular organic
compound becomes small, it is difficult to obtain any satisfactory
rate of resistance change. When this ratio becomes high, on the
contrary, the thermistor element is not only unacceptably deformed
upon the melting of the low-molecular compound, but it is also
difficult to mix the low-molecular compound with the conductive
particles. If the amount of the conductive particles is 2 to 5
times as large as the total weight of the polymer matrix and
low-molecular organic compound, it is then possible to take full
advantage of the invention. When the amount of the conductive
particles becomes small, it is impossible to make the
room-temperature resistance in a non-operating state sufficiently
low. When the amount of the conductive particles becomes large, on
the contrary, it is not only difficult to obtain any large rate of
resistance change, but it is also difficult to achieve any uniform
mixing, resulting in a failure in obtaining any reproducible
resistance value.
In the practice of the invention, milling should preferably be done
at a temperature that is higher than the melting or softening point
of the thermoplastic polymer matrix (especially the melting or
softening point +5 to 40.degree. C.). Milling may otherwise be done
in known manners using, e.g., a mill for a period of about 5 to 90
minutes. Alternatively, the thermoplastic polymer and low-molecular
organic compound may have been mixed together in advance in a
molten state or dissolved in a solvent before mixing.
Antioxidants may optionally be used to prevent thermal degradation
and oxidation of the polymer matrix and low-molecular organic
compound. For instance, phenols, organic sulfur compounds, and
phosphites may be used to this end.
A thermistor element may be obtained by pressing the obtained
mixture in a sheet form having a given thickness, and then
hot-pressing electrodes of metals such as copper, and nickel
thereon. If required, the thermistor element may be subjected to a
crosslinking treatment by means of radiation crosslinking, chemical
crosslinking using an organic peroxide, and aqueous crosslinking
due to the condensation reaction of a silanol group by the grafting
of a silane coupling agent. The electrodes may be formed
simultaneously with pressing.
The organic positive temperature coefficient thermistor according
to the invention has low initial resistance in its non-operating
state and a resistance value of about 10.sup.-3 to 10.sup.-1
.OMEGA.cm as measured at room temperature, with a sharp resistance
rise upon operation and the rate of resistance change upon
transition from its non-operating state to operating state being at
least 8 orders of magnitude greater. While no accurate upper limit
to the rate of resistance change can be found because of measuring
device constraints, it is estimated to reach at least 11 orders of
magnitude. In addition, the temperature vs. resistance curve
hysteresis is reduced.
The present invention will now be explained more specifically with
reference to examples, and comparative examples.
EXAMPLE 1
Low-density polyethylene (LC 500 made by Nippon Polychem Co., Ltd.
with a melt flow rate of 4.0 g/10 minutes, a density of 0.918
g/cm.sup.3 and a melting point of 106.degree. C.) was used as the
polymer matrix, paraffin wax (HNP-10 made by Nippon Seiro Co., Ltd.
with a melting point of 75.degree. C.) as the low-molecular organic
compound, and filamentary nickel powders (Type 255 Nickel Powder
made by INCO Co., Ltd.) as the conductive particles. The conductive
particles had an average particle diameter of 2.2 to 2.8 .mu.m, an
apparent density of 0.5 to 0.65 g/cm.sup.3, and a specific surface
area of 0.68 m.sup.2 /g.
The low-density polyethylene was previously mixed with 50% by
weight of the wax in a molten state. This polyethylene/wax mixture
was milled in a mill at 115.degree. C. for 10 minutes with the
addition thereto of the nickel powders in a weight of 4 times as
large as the mixture and dicumyl oxide as an organic peroxide in an
amount of 3% by weight of the mixture. Nickel foils of 30 .mu.m in
thickness were placed on and pressed at 110.degree. C. against both
sides of the resulting mixture, using a heat pressing machine. In
this way, a pressed assembly of 1 mm in total thickness was
obtained. This assembly was punched out into a disk shape of 10 mm
in diameter, and then heat treated at 155.degree. C. for 50 minutes
for the purpose of chemical crosslinking, thereby obtaining a
thermistor element. The structure of this thermistor element is
shown in FIG. 1. As can be seen from FIG. 1, the thermistor element
is made up of a pressed thermistor element sheet 12 comprising the
low-molecular organic compound, polymer matrix and conductive
particles, and sandwiched between nickel foil electrodes 11.
This element was heated and cooled in a thermostat to measure its
resistance value at predetermined temperatures by a four-terminal
method, thereby obtaining a temperature vs. resistance curve as
shown in FIG. 2, with solid and broken lines representing the rates
of resistance change during the rise and fall of temperature,
respectively. The room-temperature resistance (at 25.degree. C.)
was 3.times.10.sup.-3 .OMEGA., and the resistance value showed a
sharp rise at the melting point of the wax, 75.degree. C., with a
maximum resistance value of at least 10.sup.9 .OMEGA. and a rate of
resistance change of at least 11 orders of magnitude. It is also
found that the heating/cooling cycle hysteresis frequently observed
in operation using the melting point of a crystalline polymer such
as polyethylene, and polyvinylidene fluoride is considerably
reduced.
The degree of hysteresis, i.e., an index to this hysteresis was
found in the following manner.
Degree Of Hysteresis
A typical temperature vs. resistance curve showing a resistance
change during the rise of temperature is shown in FIG. 3. On this
graph, straight lines are drawn tangent to curve segments, before
and after operation, of the temperature vs. resistance curve. An
operating temperature is then given by a point of intersection of
these lines. Likewise, an operating temperature is found from a
temperature vs. resistance curve obtained during the fall of
temperature. The degree of hysteresis is defined by a difference
(absolute value) between both the operating temperatures. The
smaller the value, the more reduced the hysteresis is.
The thus found degree of hysteresis was 4.degree. C. for the
inventive element using paraffin wax, and about 15.degree. C. to
25.degree. C. for elements composed only of the aforesaid
crystalline polymers. It is thus understood that the inventive
element shows considerably reduced hysteresis.
EXAMPLE 2
A thermistor element was obtained and estimated following Example 1
with the exception that high-density polyethylene (Hizex 2100JP
made by Mitsui Petrochemical Industries, Ltd. with a melt flow rate
of 6.0 g/10 minutes, a density of 0.956 g/cm.sup.3 and a melting
point of 127.degree. C.) was used as the polymer matrix and mixed
with the wax in the same amount (weight), and milling was done at
140.degree. C. The temperature vs. resistance curve is shown in
FIG. 4. The room-temperature initial resistance value was
6.times.10.sup.-3 .OMEGA., and the resistance value showed a sharp
rise at the melting point of the wax, 75.degree. C., with a
post-operation maximum resistance value of at least 10.sup.9
.OMEGA. and a rate of resistance change being of at least 11 orders
of magnitude. From FIG. 4, it is also understood that the
resistance hysteresis is considerably reduced. In this regard, the
degree of hysteresis was 7.degree. C.
EXAMPLE 3
A thermistor element was obtained and estimated following Example 1
with the exception that microcrystalline wax (Hi-Mic-1080 made by
Nippon Seiro Co., Ltd. with a melting point of 83.degree. C.) was
used as the low-molecular organic compound. The temperature vs.
resistance curve is shown in FIG. 5. The room-temperature initial
resistance value was 3.times.10.sup.-3 .OMEGA., the post-operation
maximum resistance value was at least 10.sup.9 .OMEGA., and the
rate of resistance change was of at least 11 orders of magnitude.
From FIG. 5, it is also understood that the resistance hysteresis
is considerably reduced. In this regard, the degree of hysteresis
was 2.degree. C.
EXAMPLE 4
A thermistor element was obtained and estimated following Example 1
with the exception that behenic acid (made by Nippon Seika Co.,
Ltd. with a melting point of 81.degree. C.) was used as the
low-molecular organic compound and employed in an amount of 66%
with respect to the low-density polyethylene. The temperature vs.
resistance curve is shown in FIG. 6. The room-temperature initial
resistance value was 3.times.10.sup.-3 .OMEGA., the post-operation
maximum resistance value was at least 10.sup.9 .OMEGA., and the
rate of resistance change was again of at least 11 orders of
magnitude. From FIG. 6, it is also understood that the resistance
hysteresis is considerably reduced. In this regard, the degree of
hysteresis was 3.degree. C.
COMPARATIVE EXAMPLE 1
A thermistor element was obtained and estimated following Example 1
with the exception that carbon black (Toka Carbon Black #4500 made
by Tokai Carbon Co., Ltd. with an average particle size of 60 nm
and a specific surface area of 66 m.sup.2 /g) was used as the
conductive particles and the carbon black was milled in an amount
of 66% by weight with respect to a mixture of the low-density
polyethylene and paraffin wax. The temperature vs. resistance curve
is shown in FIG. 7. The room-temperature resistance value was
2.times.10.sup.-1 .OMEGA., the post-operation maximum resistance
value was 10 .OMEGA., and the rate of resistance change was of 1.7
orders of magnitude. As can be understood from the facts that the
room-temperature resistance value is higher than those of the
thermistor elements according to Examples 1 to 4, and the rate of
resistance change is at most 9 orders of magnitude based on the
thermistor elements according to Examples 1 to 4, this comparative
thermistor element is remarkably lacking in practicality. In this
regard, the degree of hysteresis was 5.degree. C.
When the amount of the carbon black used was increased to 100% by
weight with respect to the mixture, there was a further reduction
in the rate of resistance change, although the room-temperature
resistance could be lowered. From this it is evident that the
conductive particles having spiky protuberances are effective in
the practice of the invention.
COMPARATIVE EXAMPLE 2
A thermistor element was obtained and estimated following Example 1
with the exception that spherical nickel powders (Type 110 Nickel
Powder made by INCO Co., Ltd. with an average particle size of 0.8
to 1.5 .mu.m, an apparent density of 0.9 to 1.5 g/cm.sup.3 and a
specific surface area of 0.9 to 2 m.sup.2 /g) were used as the
conductive particles. The temperature vs. resistance curve is shown
in FIG. 8. The room-temperature resistance value was
9.times.10.sup.-2 .OMEGA., the post-operation maximum resistance
value was 18.7 .OMEGA., and the rate of resistance change was of
2.3 orders of magnitude. From this it is evident that the
conductive particles having spiky protuberances are effective in
the practice of the invention. In this regard, the degree of
hysteresis was 5.degree. C.
The room-temperature resistance values, maximum resistance values,
rates of resistance change, and degrees of hysteresis obtained in
Examples 1 to 4 and Comparative Examples 1 and 2 are enumerated in
Table 1, in which the melting points, mp, of the low-molecular
organic compounds used are also given.
TABLE 1
__________________________________________________________________________
Rate of Resistance Low-Molecular RT Max. Change Degree of Polymer
Organic Conductive Resistance Resistance (orders of Hysteresis
Matrix compound (mp) Particles (.OMEGA.) Value (.OMEGA.) magnitude)
(.degree.C.)
__________________________________________________________________________
Ex. 1 Low-density Paraffin wax Filamentous 3 .times. 10.sup.-3
10.sup.9 or greater 11 or greater 4 polyethylene (75.degree. C.) Ni
powders 2 High-Density Paraffin wax Filamentous 6 .times. 10.sup.-3
10.sup.9 or greater 11 or greater 7 polyethylene (75.degree. C.) Ni
powders 3 Low-density Microcrystalline Filamentous 3 .times.
10.sup.-3 10.sup.9 or greater 11 or greater 2 polyethylene wax
(83.degree. C.) Ni powders 4 Low-density Bhenic acid Filamentous 3
.times. 10.sup.-3 10.sup.9 or greater 11 or greater 3 polyethylene
(81.degree. C.) Ni powders Comp. Ex. 1 Low-density Paraffin wax
Carbon 2 .times. 10.sup.-1 10 1.7 5 polyethylene (75.degree. C.)
black 2 Low-density Paraffin wax Ni Spherical 9 .times. 10.sup.-2
18.7 2.3 5 polyethylene (75.degree. C.) powders
__________________________________________________________________________
RT Resistance: Roomtemperature resistance
EXAMPLES 5 to 10
Thermistor elements were obtained and estimated as in Example 1
except that such combinations of polymer matrixes with
low-molecular organic compounds as shown in Table 2 were used at
such quantitative ratios as shown in Table 2. However, milling was
done at a temperature higher than the melting or softening points
by 5 to 30.degree. C. The resultant thermistor elements were all
found to be equivalent to the thermistor elements obtained in
Examples 1 to 4 in terms of the room-temperature resistance value,
maximum resistance value, rate of resistance change, and degree of
hysteresis. In Table 2, the melt flow rates, MFRs, softening
points, sp, and melting points, mp, of the matrices and the melting
points, mp, of the low-molecular organic compounds are also given.
Regarding 12-nylon, the molecular weight, Mw, is given in place of
the melt flow rate.
TABLE 2
__________________________________________________________________________
Matrix/Organic Low-Molecular Compound Weight Polymer Matrix Organic
Compound Ratio
__________________________________________________________________________
Ex. 5 High-density polyethylene Paraffin wax HNP-10 (mp 75.degree.
C.) 1/1 (MFR 6.0 g/10 min., mp 127.degree. C.) Microcrystalline wax
Hi-Mic-2045 (mp 64.degree. C.) 1:2 by weight 6 High-density
polyethylene Paraffin wax HNP-3 (mp 66.degree. C.) 1/1 (MFR 6.0
g/10 min., mp 127.degree. C.) Microcrystalline wax Hi-Mic-3090 (mp
89.degree. C.) 1:2 by weight 7 12-Nylon (Mw = 24,000, mp
178.degree. C.) Paraffin wax HNP-10 (mp 75.degree. C.) 2/3 8 PMMA
(MFR 2.3 g/10 min., sp 110.degree. C.) Palmitic acid (mp 64.degree.
C.) 2/3 9 Polyacetal (MFR 2.8 g/10 min., mp 175.degree. C.) Oleic
amide (mp 76.degree. C.) 1/1 10 EVA (MFR 1.5 g/10 min., mp
99.degree. C.) Archic acid methyl ester (mp 48.degree. C.) 2/1
__________________________________________________________________________
High-density polyethylene: Hizex 2100JP, Mitsui Petrochemical
Industries, Ltd. 12Nylon: Ube Industries, Ltd. PMMA (polymethyl
methacrylate): Mitsubishi Rayon Co., Ltd. Polyacetal: Asahi
Chemical Industry Co., Ltd. EVA (ethylenevinyl acetate copolymer):
Nippon Polychem Co., Ltd. Paraffin wax: HNP3, HNP10, Nippon Seiro
Co., Ltd. Microcrystalline wax: HiMic-2045, HiMic-3090, Nippon
Seiro Co., Ltd. Oleic amide, palmitic acid: Nippon Seika Co., Ltd.
Arachic acid methyl ester: Tokyo Kasei Co., Ltd.
In Examples 5 and 6 where two low-molecular organic compounds were
used, a thermistor element comprising each of them was also
prepared. As a result, it was found that the thermistor element
comprising two low-molecular organic compounds has an operating
temperature different from that comprising each of them. By using
two low-molecular organic compounds, it is thus possible to control
the operating temperatures.
According to the present invention, it is possible to obtain a
positive temperature coefficient thermistor having low
room-temperature resistance and showing a large resistance change
upon operation. By use of a low-molecular organic compound it is
possible to make the temperature vs. resistance curve hysteresis
small. If low-molecular organic compounds with varying melting
points are used, it is then easy to control the operating
temperature. It is also possible to reduce the operating
temperature to 100.degree. C. or lower.
Japanese Patent Application No. 350108/1997 is herein incorporated
by reference.
While the invention has been described with reference to preferred
embodiments, it will be understood by those skilled in the art that
various changes may be made and equivalents may be substituted for
elements thereof without departing from the scope of the invention.
In addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without
departing from the essential purport thereof. Therefore, it is
intended that the invention not be limited to the particular
embodiment disclosed as the best mode contemplated for carrying out
this invention, but that the invention will include all embodiments
failing within the scope of the appended claims.
* * * * *